In recent years, from the viewpoint of the protection of the global environment, clean energy with higher safety, has been desired. Among clean energies which are expected in the future, particularly a solar cell is highly expected in terms of its cleanness, safety and easy operation.
The core to convert the sunlight put in a solar cell to electric energy is a cell. As the cell, one composed of a monocrystal, polycrystal or amorphous silicon type semiconductor is widely used. A plurality of the cells are usually wired in series or parallel, and further, they are protected with various materials for maintaining the function for a long period of time, and used as a solar cell module.
A solar cell module generally has a structure where the side of the cell hit by sunlight is covered with a tempered glass, the rear side is sealed with a back sheet, and a filer made of a thermoplastic resin (particularly an ethylene/vinyl acetate polymer (hereinafter referred to as “EVA”)) is filled in the space between the cell and the tempered glass and in the space between the cell and the back sheet, respectively.
Quality assurance of product for about 20 to 30 years is required for a solar cell module. Since the solar cell module is mainly used outside, weather resistance is required for the constituent material. Further, the tempered glass and back sheet have a role to prevent the deterioration caused by the moisture inside the module, and moisture-proof property is also required.
Although the tempered glass is excellent in not only moisture-proof property but also transparency and weather resistance, its plasticity, shock resistance, operatability and so on are low. Therefore, the application of a resin sheet, particularly a fluororesin sheet excellent in weather resistance, has been considered, instead of the tempered glass. However, the resin sheet has a problem that moisture-proof property is low as compared with the tempered glass.
To solve the above-mentioned problem, it has been proposed to provide a vapor deposition thin film of an inorganic oxide as a moisture-proof layer, on a resin sheet. For example, Patent Document 1 proposes a protective sheet having a fluororesin sheet and a resin sheet having a vapor deposition thin film of an inorganic oxide, laminated. Further, Patent Document 2 proposes a protective sheet having a vapor deposition thin film of an inorganic oxide provided on one side of a fluororesin sheet provided, and further having a stain resistant layer and/or an ultraviolet absorber layer provided, in order to improve weather resistance.
However, since the moisture-proof layer has poor adhesion to the resin sheet, when a solar cell module is constituted by providing a filler layer so as to be in contacted with the moisture-proof layer, a problem that the moisture-proof layer peels from the resin sheet, may occur. When a space is created between the resin sheet and the filler layer by the peeling, the durability of the solar cell module decreases by the immersion of moisture and the like.
To solve the above-mentioned problem, it has been proposed to provide a layer for improving the adhesion (an adhesive layer) between the resin sheet and the vapor deposition thin film of an inorganic oxide. For example, Patent Document 3 proposes to provide an anchor coat layer containing a complex mixture of an acrylic polyol and/or a polyester polyol and an isocyanate compound, between a fluororesin substrate layer and a vapor deposition thin film layer of an inorganic oxide. Further, Patent Document 4 proposes to provide a primer layer for vapor deposition, made of a resin having an amide ester moiety, which is obtained by reacting a resin containing oxazoline groups with an acrylic resin made of a poly(meth)acrylic acid and/or a copolymer of a poly(meth)acrylic acid and a comonomer, in a specific ratio, between a substrate and a vapor deposition layer made of an inorganic oxide.